Vessels, pipes, condensers and boilers used in the chemical & food processing industries, power plants, oil field operations are subject to the formation of scale, which interferes with functioning. The word “scale” when used herein includes any solid deposit formed on a solid metal surface, such as ferriferous metal surfaces, as a result of contact between the metal surface and an aqueous solution in liquid or vapor state. During use, water storage tanks, conduits, plumbing, cooling towers, process equipment, electrolysis membranes and other units develop scale which must be removed, preferably dissolved in order to maintain flow, thermal conductivity, to avoid under-deposit corrosion and hot spots that can cause boiler tube failures and to maintain the highest possible energy efficiency.
Historically, this scale was removed using a solution of hydrochloric acid. To accelerate the cleaning process, the aqueous HCl cleaner was often heated to as high as 100 degree C., but cleaning still took 4 to 12 hours or more to accomplish. The hydrochloric acid is usually present in such cleaners in a concentration range of from 2.5-15% by weight, which, upon repeated use, can be quite damaging to the metal parts of the aforementioned units.
The HCl cleaners alone often did not adequately remove silica or copper, which typically required additional additives or processes. Metallic copper deposits were generally removed by a separate step using ammoniated sodium bromate solution. Both steps resulted in higher chemical and waste disposal costs. The sodium bromate stage required a separate chemical fill and an extra rinse step. Another drawback of HCl cleaners is the high concentration of chloride ion in the cleaning solution. Chloride ion concentrations above 100 ppm or so are typically not acceptable for use in nuclear plants and certain other infrastructure due to concerns regarding possible, and difficult to predict, chloride stress corrosion damage.
It is known to utilize certain compounds or mixtures of compounds in conventional acidic HCl-based solutions that are utilized for cleaning or pickling metal surfaces to remove therefrom unwanted oxides, scale and other undesirable corrosion products. Such compounds reduce the tendency of the acidic cleaning solution to dissolve the metal surface without interfering with the cleaning operation performed by the solution. Compounds that function in this manner are generally referred to as “acid inhibitors”. In the absence of acid inhibitors, an acidic metal cleaning or pickling solution can cause significant base metal loss and also damage that can extend below the metal surface as a result of excessive hydrogen exposure which occurs in the absence of acid inhibitors.
Newer methods of cleaning or pickling metal surfaces to remove therefrom unwanted oxides, scale and other undesirable corrosion products seek to eliminate strongly acid cleaners based on HCl and instead use organic acids and/or organic acid salts at mid- to high-pH to accomplish the cleaning. An important benefit of these cleaners, referred to hereinafter as “chelating cleaners” is elimination of separate chemistries for removal of metallic copper. Metallic copper and some copper containing deposits are removed with the cleaning solution in a lower temperature second step; after lowering the temperature to about 150 degree F. and dissolving a solid, and/or while injecting a gaseous oxidizing agent. Other benefits of these cleaners include chloride-free compositions, less acidic pH, and easier waste management. Steel surfaces are left in a clean and passivated state.
The chelating cleaning solution is effective in removing undesirable deposits from metal surfaces, including those that contain silica and copper, and even metallic copper itself when using ammonia and oxidizer, but unfortunately it also tends to attack and corrode the base metal, particularly cold rolled steel. Such corrosion is very undesirable. To counteract the corrosive effects of the chelating cleaning solution, it is desirable to provide “metal loss inhibitors” for addition to the chelating cleaning solution.
It is likewise desirable to provide a metal loss inhibitor that readily disperses irreversibly throughout chelating cleaning solutions, suppresses etch and corrosion of the base metal with which it comes into contact, does not interfere with silica or copper removal, suppresses hydrogen formation and its damage and leaves little or no smut or residual film on the surface of the metal. It must also maintain effectiveness over a range of pH and iron concentrations and temperatures, with such effectiveness being sufficiently long lasting so that the metal pickling or cleaning solution need not be frequently discarded or replenished.
Further, it is desirable for cost and convenience reasons to market such metal loss inhibitor compositions in the form of concentrates that are diluted and combined with aqueous chelating cleaning solutions to prepare a metal pickling or cleaning solution. Alternatively, such concentrates are diluted to working concentrations with water and then various additional components are mixed in to prepare the working metal pickling or cleaning solutions. Inhibitor concentrates must remain stable over prolonged periods of time so that they may be safely stored until being combined with other components to form a metal pickling or cleaning solution. That is, the concentrate should remain a homogeneous solution (e.g., no phase separation or precipitation of solids) and should not deteriorate or degrade in effectiveness to a significant extent. Moreover, the solutions prepared from such concentrates must meet stringent customer requirements with respect to cost and performance (e.g., inhibition of metal etching), both immediately and over time (e.g., as iron levels in the solution increase upon continued use of the solution).
Many types of metal loss inhibitor compositions are known in the art, with several being available commercially. However, in many cases such formulations exhibit poor solubility at the high working pHs and high ionic concentrations typical of the best chelating cleaning solutions, exhibit poor rinsing, interfere with copper removal or suffer from manufacturing limitations, e.g. environmentally undesirable, hazardous or scarce raw materials. Further improvements in the art of metal loss inhibitor concentrates and metal cleaning and pickling solutions would therefore be desirable.